1. Field of the Invention
This invention relates to fences and more particularly to crash barriers for location alongside a vehicle carrying bridge or road.
2. Description of the Related Art
Many proposals have been made and crash barriers are in extensive use in many countries. GB patent 1209191 shows a construction in aluminium alloy which has been widely used. In the arrangement of GB 1209191 a series of upright posts are joined by several horizontal rails. The posts are designed to be frangible and to fail upon vehicle impact in order to absorb as much of he impact energy as possible. The failure zone in this case is at or near a welded joint between the post and a base plate and it has not proved possible to predict accurately the impact load that will cause failure. This is because there can be significant differences in the strength characteristics of the welds from post to post and also adjacent to the welds there is a heat affected zone which again may vary the strength characteristics of one post compared with another.
Thus it has been found that the posts of GB 1209191 tend to tear away their base weld upon rotation, upon impact, through about 2.degree. but that the impact load to achieve this varies from post to post by up to 30%.
Applicants are aware of one previous proposal to provide more accurate and reproducible post distortion. This is in a Japanese product (GB 2032496 A) where the rear wall of each post is formed with a shallow inward curve extending from the base of the post to a position nearly halfway of its height. This arrangement undoubtedly ensures that upon impact the post will crumple firstly at the curve. However the vertical extent of the curve and the uncertainty of its large radius of curvature ensures that there will still be variations in distortion of different posts subjected to the same impact load.
In the arrangement of GB 1209191 the rails are formed with a rearwardly facing re-entrant slot to receive the heads of four bolts secured through holes in the flat front faces of the posts. With this arrangement, the restraint to "windmilling" is dependent only upon the four bolts.
Additionally GB 1209191 provides for sheets of wire mesh to be provided as infill between the rails. However different rail sections having extruded slots are used to receive differently sized sheets of mesh. During accident damage repair it is necessary to partly dismantle the rail structure to slide out the mesh and replace it.
Thus it is an object of the present invention to provide an improved crash barrier of posts and rails in which greater control than hitherto is provided for the collapse of the posts under a known impact load.
A further object is to provide an improved method of securing the rails to the posts to reduce the possibility of windmilling.
Another object is to provide improved fixings for infill sheets to enable these to be readily replaced. A further object is to provide an improved base plate for the posts.